This invention relates to a process and a device for piecing a yarn at a spinning unit of an open-end friction spinning machine. The spinning unit has two rollers that are drivable in the same rotational direction and are arranged next to one another to form a wedge-shaped gap serving as a yarn forming region. A feeding and opening device is provided for the feeding of single fibers to the wedge-shaped gap. A yarn withdrawal device is provided for drawing off the formed yarn in the direction of the extension of the wedge-shaped gap and a selectively connectable suction device is provided for holding the forming yarn in the wedge-shaped gap. For accommodating yarn piecing without exposing the wedge-shaped gap, an already spun yarn end is brought back into the area of the wedge-shaped gap by means of an intake suction device. Subsequently, the previously interrupted feeding of fibers is resumed and the yarn is drawn off again.
Manual yarn piecing at a spinning unit of an open-end friction spinning machine is described in European Published Unexamined Application (EP-OS) No. 344 27, where only one of two rollers is formed as a so-called suction roller. This suction roller has a perforated shell and a suction insert arranged on the inside of the shell. During the piecing of the yarn, the yarn take-up spool is lifted off its operative drive, after which the already spun yarn is taken off and is shortened to a predetermined length which will suffice to return to the yarn forming region. Subsequently, the yarn, while by-passing the withdrawal rollers on the machine which continue to operate in an unchanged manner, is conveyed to a yarn withdrawal tube and drawn into the spinning unit under the effect of the low pressure of the suction device affecting the wedge-shaped gap. In the case of this device, it is provided that by means of the manual operation, the suction slot of the suction device can gradually be closed in a manner that the closing first starts in the area facing the yarn withdrawal tube. This is designed to have the result that the yarn end with the decreasing suction slot is to be drawn further into the spinning unit. As an extension of the wedge-shaped gap, another intake suction device is also provided which is supposed to hold the yarn when the suction slot is completely closed. The yarn end is then supposed to be held in a straight stretched line at a distance from the wedge-shaped gap. After the suction device is turned on again, the yarn end is supposed to place itself in a stretched position in the wedge-shaped gap, the length of the yarn end being dimensioned in such a way that the tip of the yarn end is located in the fiber feeding region. Almost simultaneously with the switching-on of the suction device, the feeding is started, after which the yarn withdrawal is started by placing the take-up spool on its drive roller, after which the pieced yarn is placed in the nip line of withdrawal rollers serving as the withdrawal device. This process does not make it possible to provide yarn piecing points or sections which are of sufficient high quality that they can also be used when the yarn is processed further. On the contrary, the yarn piecing sections must be cleaned out before a further processing of the yarn. One of the reasons is that the returned yarn end during the return is sucked to the running rollers so that the yarn end, even when it is not located in the wedge-shaped gap, receives an increased twist. In addition, the moving sequences for the turning-on and restarting of the individual steps cannot be coordinated in such a way that a yarn piecing section is obtained which, on the one hand, has sufficient stability and, on the other hand, has an appearance that corresponds essentially to the remaining yarn.
The present invention is based on the objective of providing a yarn piecing process and apparatus that is suitable for automation and provides yarn piecings of an improved quality.
These objectives are achieved according to the invention by returning the tip or extreme end section with a length that reaches beyond the yarn forming region and by switching off the suction device affecting the wedge-shaped gap while the yarn end section is being sucked back.
The fact that the yarn end section is returned beyond the yarn forming region provides a longer period of time for carrying out the individual steps of the process during repiecing so that a precise coordination and thus an improvement of the quality of the yarn piecings can be achieved. In addition, the yarn end section is not sucked into the wedge-shaped gap before the actual piecing process, especially also during the return of this yarn end section, so that, on the one hand, it is easier to effect the return and, on the other hand, it is not further twisted excessively.
In an advantageous further development according to certain preferred embodiments of the invention, it is provided that the friction rollers are stopped during the return of the yarn end section. Thus it is avoided that the yarn end is twisted before the actual piecing so that an excessive twisting is avoided.
In a further development according to certain preferred embodiments of the invention, it is provided that the suction device affecting the wedge-shaped gap is switched on after the completion of the return of the yarn end, before the switching-on of the other devices. This ensures that the yarn end section is guided into the wedge-shaped gap to the extent that this is necessary before the actual piecing process, i.e., the feeding of the fibers. It is practical in this case that simultaneously with or shortly after the switching-on of the suction device, the device for the rewithdrawal of the yarn is turned on. This ensures that the yarn end section withdrawal from the intake suction device, with its free end, reaches a precisely defined piecing position without previously being twisted or overtwisted.
In a further development according to certain preferred embodiments of the invention, it is provided that the device for the rewithdrawal of the yarn is initially driven with a withdrawal speed that differs from the operative withdrawal speed (for normal spinning conditions) and is subsequently, preferably continuously, brought to the operative withdrawal speed. This ensures, especially when a decreased withdrawal speed is provided, that longer periods of time are available for the actual piecing process so that slight time deviations do not result in significant faults.
In a further development according to certain preferred embodiments of the invention, it is provided that the switching-on of the feeding of the fibers and the restarting of the friction rollers takes place approximately simultaneously. This makes available a certain quantity of fibers already in the wedge-shaped gap before the yarn end arrives there, so that the yarn end finds fibers in the wedge-shaped gap and can be pieced onto them. It is especially practical that according to certain preferred embodiments of the invention to provide that a quantity of fibers is fed which differs from the operatively fed quantity of fibers during the piecing. It therefore becomes possible to proportion the quantity of fibers in order to avoid thick piecing points while simultaneously ensuring sufficient yarn stability. In this case, the feed quantity of fibers is adapted to the withdrawal speed of the yarn.
In a further development of the invention, it is provided that the yarn end section led back beyond the yarn forming region is guided in such a way that the yarn end is transferred to the wedge-shaped gap in the area of the feeding region of the fibers. It is especially expedient according to preferred embodiments to provide that the yarn end extending in the wedge-shaped gap during the withdrawal is guided to the wedge-shaped gap along a path which partially coincides with the path of the fed fibers. The yarn end section, and especially the tip or end of the yarn end section, will then move along one path with the fibers so that the yarn end section and the fibers are treated and affected identically, advantageously facilitating the twisting process at the piecing connection. In this case, it is expedient that the feeding of the fiber is turned on before the tip or end of the yarn end section reaches the yarn forming region. Thus a certain number of fibers are already led to the wedge-shaped gap, while another quantity is fed together with the free end of the yarn end section. In this case, it is especially advantageous that the end of the yarn end section is opened up in the form of a fiber beard or tuft before it reaches the yarn forming point. An end that was opened in this manner connects very well with the individual fibers.
In a further development of the invention, it is provided that the rewithdrawal of the yarn end section is switched on and the switching-on of the other devices is accomplished by means of a control device containing timing and control elements responsive to the end of the yarn end section passing a signal transmitting means arranged at a defined location. Thus it becomes possible to carry out the individual operating steps at exactly predeterminable times and therefore also in precisely predeterminable mutual relationships.
In a further development of the invention, a device for carrying out the process is created, where a means is provided for taking up the yarn end section from a take-up spool and transferring the yarn end section to the spinning unit, as well as a device for sucking the yarn end section into the spinning unit, and means for the rewithdrawal of the yarn end, the control of the suction device affecting the wedge-shaped gap and the switching-on of the feeding of the fibers.
In a very advantageous further development of the invention, the device for the receiving and transferring of the yarn end section, the device for making the yarn end into a beard or tuft, the device for the rewithdrawal of the yarn end, the device for the control of the suction effect of the suction device affecting the wedge-shaped gap, and the device for switching-on the feeding are arranged in a maintenance cart that can be moved along the open-end friction spinning machine and selectively positioned adjacent respective spinning units to be serviced. This makes it possible that, on the one hand, the whole piecing process takes place mechanically so that errors by the operating personnel can be avoided. In addition, it is achieved that the devices interacting with one another in certain time sequences are arranged in the maintenance cart so that the precise adjustment of these devices with respect to one another will lead to uniform results at all spinning units. Another advantage is that the devices representing increased expenditures in each case only have to be available once.
Further objects, features, and advantages of the present invention will become more apparent from the following description when taken with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.